* Increase the default thumbnail size from small (150x150) to medium (180x180).
* Change the mobile layout to use three posts per row instead of two for small thumbnails.
Parent/child posts are still 150x150 to avoid taking up even more space above posts.
Factor out thumbnail galleries into a PostGallery component.
This changes the html structure so that post galleries on all pages are
always wrapped in a `.posts-container` class. This fixes an issue with
thumbnails on the pool show page not being aligned correctly on mobile,
like they are on the post index page. This also affected thumbnail
galleries on other pages, like wiki pages and user profiles.
Add rel=nofollow to various internal search links to prevent Google from
attempting to crawl things like `<tag> status:deleted` or `approver:<name>`
searches.
* Convert notices from helpers to partials.
* Eliminate PostSets::PostRelationship class in favor of post_sets/posts template.
* Eliminate COUNT(*) queries when calculating the number of child posts.
* Eliminate redundant parent load and parent exists queries.
